As illustrated in FIG. 11, a steering apparatus for an automobile is constructed such that rotation of a steering wheel 1 is transmitted to an input shaft 3 of a steering gear unit 2, and as the input shaft 3 rotates, a pair of left and right tie rods 4 are pushed or pulled, which applies a steering angle to steered wheels (front wheels). The steering wheel 1 is supported by and fastened to the rear end section of a steering shaft 5, and that steering shaft 5 is inserted in the axial direction through a steering column 6, and is supported by this steering column 6 so as to be able to rotate freely. Moreover, the front end section of the steering shaft 5 is connected to the rear end section of an intermediate shaft 8 by way of a universal joint 7, and the front end section of this intermediate shaft 8 is connected to the input shaft 3 by way of another universal joint 9. In the example in the figure, an electric power-steering apparatus, having an electric motor 10 as an auxiliary power source, is assembled in order to reduce the force required for operating the steering wheel 1. Therefore, the front end section of the steering shaft 5 is connected to the input side of this electric power-steering apparatus, and the output shaft of this electric power-steering apparatus and the rear end section of the intermediate shaft 8 are connected by way of the universal joint 7 so as to transmit torque freely.
Cross universal joints are used as the universal joints 7, 9 that connect the steering shaft 5, intermediate shaft 8 and input shaft 3, which are rotating shafts that are assembled in this kind of automobile steering apparatus and that are not on the same line as each other. Various kinds of structure are known for this kind of cross universal joint, for example, as disclosed in JPH 06-280889 (A), JPH 08-270669 (A), JPH 11-325098 (A), and JP 2009-299706 (A). FIG. 12 illustrates an example of conventional structure as disclosed in JPH 08-270669 (A).
The universal joint 11 that is illustrated in FIG. 12 is constructed by joining a pair yokes 12a, 12b by way of one joint spider 13 so as to able to freely transmit torque. These yokes 12a, 12b are made by performing pressing or forging of a metal material, and each yoke 12a (12b) comprises a base section 14a (14b) and a pair of linking arm sections 15a (15b). A pair of circular holes 16a (16b) is formed concentric with each other on the tip end of each of the linking arm sections 15a (15b). The joint spider 13 is constructed so that four shaft sections 17 are such that the center axes of adjacent shaft sections 17 are orthogonal with each other. These shaft sections 17 are supported on the inside of the circular holes 16a, 16b by way of cup shell type radial needle bearing 18, to form a universal joint 11.
When a vehicle, in which a steering apparatus is mounted with these kinds of universal joints 7, 9, 11 assembled, is involved in a collision accident, or when a steered wheel rides up on a curb by mishandling, an impulsively excessive torque may be applied to the universal joints 7, 9, 11 from the steering gear unit 2 side. Due to this kind of impulsively excessive torque, there is a possibility that all or part of these component members will be damaged, or that safe continuous operation of the vehicle will be obstructed. For example, the linking arm sections 15a (15b) may become deformed, and concentricity of the circular holes 16a (16b) that are formed on the tip end sections of these linking arm sections 15a (15b) may be impaired. In the case that this concentricity is impaired, problems occur in that the function of the radial needle bearing 18 is impaired, and it becomes easy for each of the parts to wear out. In such a case, even though the user may take the vehicle to a repair shop, if the repairperson is inexperienced and depending the extent of the damage, there is a possibility that the problem with the universal joints 7, 9, 11 will not be found. Furthermore, in the case that an impulsively excessive torque is applied, there is a possibility that the circular holes 16a (16b) of the linking arms 15a (15b) will be pressed and expanded, allowing the radial needle bearings 18 of the joint spider 13 to fall off from the circular holes 16a (16b), impairing the function as a cross universal joint.